1. Field of the Invention
The present invention relates generally to semiconductor imagers employing color filters.
2. Description of the Related Art
Known semiconductor image sensors include a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. Regardless of the type of imager, the photo-generated charges accumulated in the photoconversion devices from each pixel are transferred to additional circuitry to form an electrical output image. Known semiconductor imagers include complementary metal-oxide semiconductor (CMOS) imagers and charge-coupled device (CCD) imagers. In CCD imagers, the accumulated charges of multiple pixels in the pixel array are read sequentially through an output amplifier. In CMOS imagers, the accumulated charge of each pixel is read individually using multiple transistors dedicated to each pixel. Although it will become clear that the present invention is not limited to any particular type of semiconductor imager, in order to provide some context for the invention, an exemplary CMOS image sensor is now described with reference to FIG. 1.
The illustrated imager shown in FIG. 1 includes a conventional pixel cell 100. Pixel cell 100 typically includes a photodiode 4 having a p-region 8 and an n-region 6 in a p-substrate 2. The pixel 100 also includes a transfer transistor with associated gate 20, a floating diffusion region 16 formed in a more heavily doped p-type well 12, and a reset transistor with associated gate 18. Photons striking the surface of the p-region 8 of the photodiode 4 generate electrons that are collected in the n-region 6. From n-region 6, the accumulated charge is read out through circuitry comprising plugs 24 and conductive features 26 formed in successive transparent insulating layers 28 according to the desired characteristics of pixel cell 100. Pixel cell 100 further comprises trench isolation regions 10 formed in p-substrate 2 used to isolate adjacent pixel cells. A color filter 30 is typically formed on top of CMOS pixel 100 and substantially over photodiode 4. A microlens 32 may be provided over color filter 30 to direct incident light towards photodiode 4.
A color filter 30 is typically a polymer-based film sensitive to different wavelengths in the visible spectrum. Each pixel of a CMOS imager is covered with a color filter, typically a red, blue, or green filter. These color filters together comprise a color filter array (“CFA”) arranged in a specific pattern. This pattern, or sequence, of filters can vary, but the “Bayer” CFA pattern, has been widely adopted. A typical red-blue-green Bayer CFA pattern consists of rows of alternating red and green color filters and alternating blue and green color filters as shown in FIG. 2a. Each color filter 30 in Bayer CFA 50 corresponds to one pixel in an underlying CMOS imager. FIG. 2b shows another common Bayer CFA that uses cyan, yellow, and magenta color filters 30.
Using color filters in semiconductor imager applications presents several undesirable constraints for high volume manufacturing. Examples of these constraints are the difficulty in achieving uniform chemical properties in the polymer color filters, uniform filter thickness, stability of the color filters in the semiconductor imager and uniform positioning of color filters in a semiconductor imager, e.g., uniform proximity to the photodiode. Conventional polymer color filters also complicate the manufacturing process because they are separate components that must be integrated into the semiconductor imager product.
There is, therefore, a need for a semiconductor imager that does not utilize conventional polymer color filters and as a result does not suffer the shortcomings associated with these color filters. A method of fabricating such a semiconductor imager is also needed.